# File lib/em/deferrable.rb, line 70
    def errback &block
      return unless block
      @deferred_status ||= :unknown
      if @deferred_status == :failed
        block.call(*@deferred_args)
      elsif @deferred_status != :succeeded
        @errbacks ||= []
        @errbacks.unshift block # << block
      end
      self
    end